Can be used to refer to all of Coach/Economy Class on a plane but usually used to highlight those passengers on an flight who, through their behavior, make it very apparent that they aren't familiar with how to get around at the airport, get through security or get on a plane without delaying or inconveniencing everyone else.
Typically they are from flyover country.

"We left 30 minutes late because there were a bunch of people back in Kettle Class who couldn't figure out how to get in their seats"